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[1] We present a new investigation of the variability in the metallic calcium ion concentration near the mesopause region, and its relation to the electron concentration during summer and winter seasons at the Arecibo Observatory. During the summer months the ion layer is broader, extending to 87-88 km compared with winter months where it occurs above this altitude around midnight. The concentration maximizes to $200 ions cm À3 around 90-95 km close to midnight during the summer. However, for the winter months, the peak occurs during the early morning hours in thin descending layers above 98 km. Summer to winter variation in the calcium ion to electron ratio displays an average value of $0.15 and 0.05 during these seasons, respectively. A good correlation between them suggests that Ca + densities are directly related to the strength of the Sporadic E, which is stronger in the summer. The average abundance of ions is 5.7 Â 10 7 cm À2 and 4.6 Â 10 7 cm À2 during summer and winter months respectively, while that for electrons is 1.2 Â 10 10 ions cm À2 and 5.8 Â 10 9 ion cm À2 for these seasons. Both Ca + and N e display strong descending layers at different altitudes during summer and winter. Calcium ion lifetimes against neutralization are a factor of two lower during the summer than in the winter months around 90 km but similar at altitudes exceeding 95 km.Citation: Raizada, S., C. A. Tepley, B. P. Williams, and R. García (2012), Summer to winter variability in mesospheric calcium ion distribution and its dependence on Sporadic E at Arecibo,

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Since 1980, we have observed the thermospheric neutral wind at the Arecibo Observatory using a Fabry‐Perot interferometer to measure the O(1D) 630 nm emission. Burnside and Tepley (1989) examined the first 8 years of this extended data set and found that there were no significant or systematic solar cycle influences on the magnitude or direction of the neutral wind field, nor on its horizontal gradients. Such affects have been observed previously at other locations around the globe, and their absence at Arecibo may have been due to the limited data set. Thus, we have extended the period of acquisition and analysis of our neutral wind measurements to include nearly three complete solar cycles (or approximately 30 years) and will present our results within the framework of the earlier work. While the earlier conclusion that no major systematic solar cycle influence on the neutral winds at Arecibo generally remains intact, we did find a slight increase in wind magnitude and a gradual, yet consistent rotation of the thermospheric neutral wind vector from a general southeast to a more eastward flow during 30 years of observation. We explain the magnitude and directional variations in terms of long‐term changes in the density and temperature of the upper atmosphere and their possible dissimilar influences on each wind component that appear as a rotation of the neutral wind vector.

[1] We describe a new technique to derive neutral atomic oxygen density, [O], in the upper thermosphere using coincident incoherent scatter radar (ISR) and airglow emission observations from Arecibo Observatory. The technique exploits the nearly resonant charge exchange coupling between neutral and ionized hydrogen and oxygen that serves as the dominant chemical source and sink of protons near and above the F region peak. Under charge exchange production and loss of H + , the proton continuity equation can be solved for [O] using twilight H density profiles derived from measured H emission brightness at 656.3 nm together with ion density, temperature, and flux obtained simultaneously by the Arecibo ISR. We present both equilibrium and nonequilibrium solutions for [O] between 500 and 1500 km during a single quiescent nighttime interval under moderate solar activity. Comparisons with theoretical expectations and with MSIS-model calculations of O density are used to identify the altitude and local time extent over which the technique is justified. These comparisons generally support technique validity between $600 and 800 km, where sufficient reactant densities are present to validate the charge exchange formulation of the continuity equation. Equilibrium solutions for [O] near 650-700 km exhibit excellent agreement with MSIS estimates before midnight, but deviations arising from ion transport become increasingly significant both above this height and as dawn approaches. Incorporation of measured proton flux gradients into the nonequilibrium solutions improves agreement between the derived and modeled estimates significantly after midnight, while the minor nonequilibrium contributions during several hours before midnight lend additional support for the presence of charge exchange equilibrium.